1. Field of Invention
The present invention relates to a distance measurement system, and more particularly to a phase measurement method.
2. Description of Related Art
Refer to FIG. 1. FIG. 1 illustrates a conventional phase-shift laser distance measurement system 100. The emitter 10 emits two optical signals T(t) wherein one of the optical signals T(t) is received by the first receiver 20A and another of the optical signals T(t) is emitted to the target object 5 to generate a reflective optical signal received by the second receiver 20B. The optical signal T(t) received by the first receiver 20A is mixed with the mixed signal H(t) to provide a reference signal Ref(t). The reflective optical signal received by the second receiver 20B is mixed with the mixed signal H(t) to provide a target signal Sig(t) wherein the mixed signal H(t) might come from a mixer. The first phase detector 30A detects the phase of the reference signal Ref(t), and the second phase detector 30B detects the phase of the target signal Sig(t). The processing unit 40 calculates the distance between the target object 5 and the measurement system 100 by the phase difference between the reference signal Ref(t) and the target signal Sig(t).
In the prior art, the avalanche photodiode (APD) is used to be the photoelectric converter of the first receiver 20A and the second receiver 20B such that the received optical signal is converted into a corresponding outputted electric signal. However, the luminous intensity received by the avalanche photodiode may vary with ambient conditions, such as the reflection of the target object surface, the distance, the temperature, the atmosphere etc. Therefore, the amplitude of the corresponding outputted electric signal, generated by the light beam signal received by the avalanche photodiode, is too unstable to measure the distance accurately.